Pyroelectric detectors have been widely used in the past for detecting pulsed energy radiation and for measuring the peak intensity of such radiant energy. The pyroelectric detector generally includes a pyroelectric crystal having a front surface, through which the incident pulse is received, and a rear surface. When energy is received, a voltage is developed between a terminal on the front face of the detector and a terminal on the rear face of the detector, as described below. One of these terminals is normally grounded and the other supplies a signal to a measuring device. The rear surface is, generally, thermally "grounded" by a heat "sink" which rapidly absorbs the heat energy collected by the crystal during detection.
As is well known in the art, a pyroelectric crystal produces a charge which is proportional to the energy of the incident pulse. This charge is generally converted by the internal capacitance of the crystal into a voltage across the terminals of the crystal which is proportional to the charge. When relatively high energies (in the order of 1 Joule) are to be detected, the internal capacitance of the crystal is insufficient and, therefore, an external capacitor must be connected parallel to the crystal in order to properly convert the charge into voltage. The combined effect of the internal capacitance and the external capacitor will be hereinafter referred to as the total capacitance of the detector.
Normally, it is desirable to detect repetitive radiation events and, therefor, the detector must be rapidly discharged after each detection. If the detector is not completely discharged between consecutive detections, the detector output may be biased by a gradually increasing residual voltage (generally referred to as base-line shift) which reduces the reliability of the pulse-energy measurement. Rapid discharge is normally achieved by discharging the ungrounded terminal of the crystal through a discharge resistor. It is appreciated that the effective detector discharge time is proportional to the RC time constant of the capacitor/discharge resistor combination. Therefore, in order to achieve a high detection repetition rate a relatively low discharge resistance (typically 1 M.OMEGA.) discharge resistor should be used.
But when the effective discharge period is very short, the detector may be prematurely discharged, i.e. discharged before the pyroelectric crystal had been charged to the desired peak voltage level. This problem becomes more apparent when the duration of the detected pulses is long compared to the time intervals between pulses. It is appreciated that even when the detected energy pulses are very short, the resultant build-up of charge in the crystal is not immediate because the build-up rate is limited by the rate of diffusion of heat through the detector's front surface and any radiation absorbent material thereon. Therefore, for many practical uses, the discharge resistance cannot be as low as may be desired for high repetition rates.
The typical output pulse from existing pyroelectric detectors is characterized by a sharp rise to a peak voltage followed by an exponential decay to a low voltage. This pattern does not reproduce the wave-shape of the detected energy, due to the discharge method. The typical output of conventional detectors is also characterized by a drifting baseline which becomes more pronounced when the discharge resistance is increased.
There is thus an inherent conflict between the requirements for high detectability which require a very large resistance, fast reset (for high repetition rates) which requires a very low resistance and low inter-pulse drift which requires an intermediate resistance.